We can always assume everything (as long as it doesn't outright contradicts canon), though - otherwise, all those speculation threads around would have been meaningless. It's just that there is no guarantee that those assumptions are, or will ever be canon. Chronicle was meant to offer an explanation, but it wasn't a history book like real life history books (not that history books detailed everything that happened either) per se. Essentially, it's just a guide / material book, not too different from UVG, WCx Manual, and such - they only describes the events Blizzard want them to, not everything happened in the events mentioned in those books. Even ignoring this, there are a lot of details or events not mentioned in Chronicle, it doesn't mean those never happened or are no longer canon.
In this particular case, it's true that Chronicle only named four Old Gods and mentioned the battle to destroy / imprison those four, but it has never explicitly stated anything in the line of "Those were the only one, there weren't any other!". Thus, technically, Blizzard can add millions other Old Gods and battles as part of the ordering of Azeroth, and it'd still be considered as expanding canon instead of a retcon.